A photonic crystal fibre is a special form of optical fibre. Optical fibres are used in many fields including telecommunications, laser machining and welding, laser beam and power delivery, fibre lasers, sensors and medical diagnostics and surgery. They are typically made entirely from solid transparent materials such as glass and each fibre typically has the same cross-sectional structure along its length. The transparent material in one part (usually the middle) of the cross-section has a higher refractive index than the rest and forms an optical core within which light is guided by total internal reflection. We refer to such a fibre as a standard fibre.
Single-mode optical fibres are preferred for many applications because of their superior wave-guiding properties. However, even so-called single-mode optical fibres do not generally offer adequate control over the polarisation of propagating light. A single-mode fibre is so called because it supports only one transverse spatial mode at a frequency of interest, but that spatial mode exists in two polarisation states; that is two degenerate modes that are polarised in orthogonal directions. In real fibres, imperfections will break the degeneracy of those modes and modal birefringence will occur; that is, the mode propagation constant β will be slightly different for each of the orthogonal modes. Because the modal birefringence results from random imperfections, the propagation constants will vary randomly along the fibre. In general, light introduced into the fibre will propagate in both modes and will be coupled from one to the other by small bends and twists in the fibre. Linearly polarised light will be scrambled into an arbitrary polarisation state as it propagates along the fibre.
In order to maintain the polarisation of a mode in a standard fibre, birefringence can be deliberately introduced into the fibre (so that the effective indices of the two polarisation modes are different) in order to render insignificant the effects of small imperfections. If light is linearly polarised in a direction parallel to one of the optic axes of the fibre then the light will maintain its polarisation. If it is linearly polarised at some other angle, the polarisation will change, as the light propagates down the fibre, from linear to elliptical to linear (not parallel to the starting polarisation) to elliptical and back to linear again, with a period known as the beat length, LB, where       L    B    =            2      ⁢      π                                      β          x                -                  β          y                          and βx and βy are the propagation constants of the orthogonal modes. That variation is a consequence of a phase difference between two orthogonal components of the mode, which results from the difference in their propagation constants. The shorter the beat length, the more resilient is the fibre to polarisation-scrambling effects. Typically, conventional polarisation-preserving fibre has a beat length of the order of a millimeter. The strength of birefringence can also be represented by the parameter       β    =                                                                β              x                        -                          β              y                                                          K          0                    =                                            n            x                    -                      n            y                                        ,where       k    0    =            2      ⁢      π        λ  where λ is the wavelength) and nx and ny are the refractive indices seen by the orthogonal modes.
In the last few years a non-standard type of optical fibre has been demonstrated, called the photonic-crystal fibre (PCF). Typically, this is made from a single solid, and substantially transparent, material within which is embedded a periodic array of air holes, running parallel to the fibre axis and extending the full length of the fibre. A defect in the form of a single missing air hole within the regular array forms a region of raised refractive index within which light is guided, in a manner analogous to total-internal-reflection guiding in standard fibres. Another mechanism for guiding light is based on photonic-band-gap effects rather than total internal reflection. Photonic-band-gap guidance can be obtained by suitable design of the array of air holes. Light with particular propagation constants can be confined to the core and will propagate therein.
Photonic-crystal fibre can be fabricated by stacking glass canes, some of which are capillaries on a macroscopic scale, into the required shape, and then holding them in place while fusing them together and drawing them down into a fibre. PCF has unusual properties such as the ability to guide light in a single-mode over a very broad range of wavelengths, and to guide light having a relatively large mode area which remains single-mode.
Birefringence can be produced by several mechanisms. It can be caused by the anisotropic nature of the polarizability of a material; i.e. by anisotropy at an atomic level. It can be caused by the arrangement of elements of a material structure at a scale larger than atomic; that phenomenon is known as form birefringence. It can also be caused by mechanical stress; that phenomenon is known as stress birefringence or the photo-elastic effect. In standard fibres, form birefringence is achieved by changing the shape of the fibre cross-section; for example, by making the core or cladding elliptical. Birefringence in a weakly-guiding fibre is generally rather weak (B˜10−6). Stress birefringence can be induced by inserting rods of borosilicate glass on opposite sides of the fibre core in the fibre pre-form. Variation in the location and shape of the borosilicate rods can induce different levels of birefringence. Stress-induced birefringence permits B˜10−4.
The methods used to produce birefringence in standard fibres, and thus to produce standard polarisation-preserving fibres, are, in general, not directly suitable for use in photonic-crystal fibre.
An object of the invention is to provide a photonic crystal fibre which is birefringent so that the fibre can be used as a polarisation-preserving fibre. Another object of the invention is to provide a method of producing such a fibre.